Fix a race condition on parallel lookup of removed files

Test case:
1) mount the FS
2) create a file in it
3) remove that file using any other means (another parallel geesefs, aws-cli, Web UI...)
4) wait 1 minute (cache TTL)
5) without first re-reading the directory with that file, run:
   `(for i in {1..20}; do echo "ls -l mnt/dir/abc &"; done; echo wait) | bash`
   where mnt/dir/abc is your deleted file
6) observe geesefs crash with `panic: dir.removeName(abc) but child not found: 2`

Author: vitalif

internal/dir.go

@@ -926,10 +926,18 @@ func (parent *Inode) removeAllChildrenUnlocked() {
 }
 
 // LOCKS_EXCLUDED(parent.fs.mu)
+// LOCKS_EXCLUDED(parent.mu)
+// LOCKS_EXCLUDED(inode.mu)
 func (parent *Inode) removeChild(inode *Inode) {
 	parent.mu.Lock()
 	defer parent.mu.Unlock()
 
+	l := len(parent.dir.Children)
+	i := sort.Search(l, parent.findInodeFunc(inode.Name))
+	if l <= 2 || i >= l || parent.dir.Children[i] != inode {
+		return
+	}
+
 	inode.mu.Lock()
 	defer inode.mu.Unlock()